Aging and fatigue of ferroelectrets: combined experimental and theoretical study

本课题的目的是对多孔铁驻极体的机电性能的稳定性进行实验和理论研究。特别是揭示了影响铁电驻极体老化和疲劳的三种不同机制。首先，在多孔聚合物和封闭体聚合物中都存在粘弹性效应。它导致材料的力学行为依赖于时间和温度，从而影响驻极体的机电性能。为了表征不同温度下不同微观结构的非荷电铁驻极体的粘弹性行为，我们首先对其进行了实验和数值研究。所获得的知识随后将被传递到带电铁驻极体的研究中。其次，由于多孔聚合物的导电弛豫，即载流子在介质中的扩散/漂移，界面电荷随时间衰减。这种导通感应放电行为也与温度有关。由于界面电荷对铁电驻极体的压电性起着至关重要的作用，因此导电感应放电对其机电性能的稳定性起着至关重要的作用。考虑这一流量的实验研究和理论模型也在计划之中。第三，在机械加载过程中，当电场由于构型变化而达到击穿电场时，界面电荷也会发生变化。当界面充电水平较高和/或机械载荷较大时，更有可能发生击穿引起的放电。这三个效应将在本项目中通过实验和理论研究相结合的方式进行研究，其结果将指导长期和机电稳定的铁驻极体的设计。

The objective is a combined experimental and theoretical study on the stability of the electromechanical properties of porous ferroelectrets. In particular, three different mechanisms which play influential roles in aging and fatigue of the ferroelectrets will be revealed. Firstly, the viscoelastic effect appears in both the porous polymer and the blocking bulk polymer. It leads to time- and temperature-dependent mechanical behavior of the material and thus influences the electromechanical properties of the ferroelectrets. To characterize the viscoelastic behavior uncharged ferroelectrets with different microstructure at different temperatures will first be experimentally and numerically studied. The gained knowledge will be then passed on to the study of charged ferroelectrets. Secondly, the interface charge decays with time due to the conductive relaxation of the porous polymer, i.e. the diffusion/drifting of charge carriers through the dielectric media. This conduction-induced discharge behavior is also temperature dependent. Since the interface charge is essential for the piezoelectric effect of ferroelectrets, the conduction-induced discharge plays an important role in the stability of their electromechanical property. Experimental investigations and theoretical model to take this discharge into account are planned. Thirdly, the interface charge can also change during the mechanically loading, when the electric field reaches the electric breakdown field due to configuration change. This breakdown-induced discharge is more likely to happen when the interface is charged on a high level and/or the mechanical loading is large. These three effects will be investigated in the present project through combined experimental and theoretical studies, results of which should guide the design of long-term and electromechanically stable ferroelectrets.